Is it permissible for Muhammad to marry the questioner's sister, knowing that their mother breastfed her brother Najib, but did not breastfeed him?

If your mother breastfed "Najeeb," then he is a brother to her daughters only through fosterage, and not to the other sons of the daughters who were not breastfed by her. Therefore, it is permissible for Muhammad to marry your sister unless there is another legitimate reason for fosterage between them. The prohibition is established by five full breastfeedings.